A) Field of the Invention
This invention relaters to a semiconductor light emitting device and a method of manufacturing a semiconductor light emitting device.
B) Description of the Related Art
Generally Au is used as a substrate metal pattern used in a power package for high current application such as a light emitting diode (LED) package (semiconductor light emitting device) for a headlamp of a vehicle because of its chemical stability. Moreover, a high efficiency LED package is desired as a result of a need for energy conservation and so it has been suggested to use Ag or Ag alloy, which has high reflectivity in a visible light range, for a substrate metal pattern (for example, refer to Japanese Laid-open Patent No. 2007-266343, Japanese Laid-open Patent No. 2008-091831, Japanese Laid-open Patent No. 2008-135588, Japanese Laid-open Patent No. 2008-091818, Japanese Laid-open Patent No. 2008-072013, Japanese Laid-open Patent No. 2008-010591 and Japanese Laid-open Patent No. 2008-053564).
A sulfide film is formed on an Ag surface by exposing the Ag surface in a normal atmosphere or in a sulfur atmosphere. The sulfide film tends to become thicker as sulfuration progresses. Japanese Laid-open Patent No. H11-190692 discloses relationships between sulfuration testing time and a thickness of silver sulfide film. Referring to a result of the test in an atmosphere of hydrogen sulfide and nitrogen dioxide mixture gas according to Japanese Laid-open Patent No. H11-190692, a thickness of the silver sulfide film increases in proportion to increase in the testing time. It suggests that sulfide ions absorbed on an Ag surface diffuse inward in silver sulfide and reactions begin at an interface between silver sulfide and Ag to increase the thickness of the silver sulfide film. Moreover, Japanese Laid-open Patent No. H11-190692 teaches that the thickness of the silver sulfide becomes several micrometers.
Silver tarnishes when it is exposed to air and forms a black layer of silver sulfide. Silver sulfide has very low reflectivity in a visible light range and is not suitable for a reflective film of a LED substrate. Therefore, it is common to restrain sulfuration of Ag by applying a protection coat on an Ag film.
FIG. 10 is a schematic cross sectional view showing a structure of a conventional reflective film.
In case of using Ag as a reflective film, it is unusual to use Ag by itself but common to use a layered structure. For example, a reflective film 50 has a layered structure of an adhesion layer 52, a barrier layer 53, a reflective layer 54 and a protection layer 55, which are stacked in this order above a substrate 51. Ti is used for the adhesion layer 52, and One of Ni, Pd, Pt, TiN, etc. is used for the barrier layer 53. Ag is used for the reflective layer 54 and the airtight protection layer 55 made of organic material such as resin or of non-organic material such as SiO2, TiO2, etc. for restraining sulfuration of Ag of the reflective layer 54 is formed on the reflective layer 54.
In the conventional reflective film structure shown in FIG. 10, an upper surface of the Ag pattern (reflective layer) 54 is covered with the protection layer 55; however, a surface of Ag (reflective layer) 54 is exposed at an edge of the reflective layer 54. Therefore, if exposed in the air, a sulfide layer 56 is formed and grown from the surface of the edge as shown in FIG. 11, and an area of a substrate horizontal part (highly reflective region) 57 decreases.
As the thickness of the sulfide layer 56 becomes thicker, the reflected luminous flux from the reflective film 50 of the LED package decreases. Especially in case of sealing an LED element with resin containing fluorescence material, incident light to the reflective film becomes strong by scattering effect of the fluorescence material. Therefore, influence of sulfuration on the reflected luminous flux from the reflective film 50 of the LED package becomes larger.
It is possible to cover the edge of the reflective layer 54 with the protection layer 55 after forming the reflective layer 54 made of Ag for restraining the formation of the sulfide layer from the edge of the reflective layer as shown in FIG. 12. However, in order to adopt the structure shown in FIG. 12, further pattering is necessary after depositing the protection layer 55 after patterning the adhesion layer 52, the barrier layer 53 and the reflective layer 54, and it increases a number of manufacturing steps, makes manufacturing management complex and rises a manufacturing cost.